bAgu is a common Telugu word meaning 'good', 'well being' etc... I didn't find the cognates in SD-I languages. Dravidian Etymology Dictionary(DED) mentions Tamil word vAkku meaning perfection is a cognate. This must be an exception where a Telugu word has 'ba' instead of 'va' while Tamil shows 'va'. Generally, Tamil and Telugu have 'va' sound where Kannada/Tulu have 'ba'. Unfortunately, equivalent Kannada and Tulu words are not available. So is the case with Malayalam, according to DED.
But today I was reminded of a word in my variant of Malayalam. Though standard dialect of Malayalam uses nallad for good; we say pAG. eg. ad pAGilla; nI maTTed nOkkiko(That is not good; you check the other one). However, we use pAG mostly with non-human objects and nalled with humans.
Update: Commenter Mallinath has come up with few more exceptions to 'ba', 'va' rule. But of the three words he mentioned two appeared to be Indo-Aryan import to Dravidian languages, baNDi, baNDa and the remaining one, bratuku, appears to be a borrow into Telugu from Kannada(I assume because the same noun form is said to be a borrow into Tamil from Kannada, according to DED*).
While ruling out all these exceptions, I became suspicious of the word bAgu. Then suddenly I remembered a stupid dialogue from a horrible Hindi movie. "mErE man ko bhAyA; mE kutta kAT ke khAyA" (My mind felt good; I cut a dog and ate). Indeed, Telugu bAgu and Malayalam pAG are borrows from IA languages. I have already mentioned that a big chunk of IE speakers assimilated with Dravidians. I consider the language of these people was Magadhi Prakrit. Therefore, any exceptions to 'va', 'ba' rule (which I firmly believe is geographical for Dravidians) must be checked from Magadhi Prakrit angle.
* Please find the links to Dravidian dictionaries at the sidebar(Starling and Digital Dictionaries of Asia).